1. Field of the Invention.
The present invention relates to an apparatus for measuring magnetic fields that change at extreme low frequency using a SQUID magnetometer that has a superconducting flux transformer for inductively coupling a measuring signal into a sensor with a direct current SQUID and that contains a device for modulating the measuring signal.
2. Description of the Related Art.
SQUID (super-conducting quantum interferometer) magnetometers can be built with high magnetic field sensitivity, for example, on the order to magnitude of 10.sup.-14 Tesla x (Hertz).sup.-1/2 A suitable magnetometer generally contains a SQUID sensor and a flux transformer for indirectly coupling the magnetic field to be detected into the SQUID sensor.
The so-called 1/f noise determines the outermost limit of sensitivity for this type of magnetometer. This limit is also inversely proportional to frequency (cf. for example "Journal of Low Temperature Physics", Vol. 51, Nos. 1/2, 1983, pages 207 to 224).
The publication "SQUID Superconducting Quantum Interference Devices and their Applications" (Proc. Int. Conf. on Supercond. Quantum Devices, Berlin 1976), 1977, pages 439 to 484, describes an apparatus for underwater communication. This device can also detect magnetic fields with extremely low frequencies from 30 to 3000 Hz. This device, inter alia, has a RF-SQUID as sensor. The sensor is located together with a super-conducting flux transformer in a cryostat (cf. page 449, FIG. 3). This flux transformer comprises a field coil that serves as an antenna, also called a detection loop, which is connected with a coupling coil to couple the measuring signal into the SQUID. These superconducting parts of the device can be developed, in particular, as thin-film structures. The super-conducting part of the device comprising the flux transformer and the SQUID sensor is connected with low noise level electronics connected downstream.
It is known that the level of noise can be further lowered by applying direct current (D) SQUIDS to such measuring devices (cf. for example "Applied Physics Letters", Vol. 40, No. 8, Apr. 15, 1982, pages 736 to 738).
However very small quasi-static magnetic fields that have frequencies of, for example, 0.01 Hz require measuring times in the range of a second and cannot be measured with known devices with sufficient sensitivity because of the 1/f noise of the SQUID sensor begins below approximately 1 Hz. This situation is particularly true for fields from remote sources that cannot be influenced, for example by modulation, and are not homogeneous. Then magnetometers can be used in a known manner by generating a modulation signal through vibration of a detection coil. The modulation can also be effected in an analogous manner through motion of a magnetizable sample relative to a detection coil (cf. for example the book by B.I. Bleaney and B. Bleaney: "Electricity and Magnetism", 3rd edition, 1976, page 187). The extremely low frequency range mentioned above requires that the characteristic length of the field variation must larger than the dimensions of the SQUID sensor or the coolant container surrounding it. Moreover, a sufficiently low flux noise cannot be achieved with the ferromagnetic material provided for a moving sample.
Electronic circuit measures are known for lowering the minimum measuring frequency limit of the 1/f noise of a DC-SQUID. See "Applied Physics Letters", Vol. 49, No. 20, Nov. 17, 1986, pages 1393 to 1395. These measures, however, only suppress the noise component caused by fluctuations of the critical current of the DC-SQUID. The noise components corresponding to magnetic flux noise cannot be diminished through application of the known circuit measures.